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Thread: LED

  1. #1 LED 
    Forum Freshman AlphaParticle's Avatar
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    Pretty easy question, again!
    What is an LED? and what is the difference between it and normal light bulbs that you would find in a house hold light??????


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  3. #2  
    Veracity Vigilante inow's Avatar
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    It's a light emitting diode. It's brighter and uses much much less energy... is more efficient b/c it doesn't lose so much as heat like incandescents.


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  4. #3  
    Forum Freshman dcOSU's Avatar
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    This may be too in depth for what you are asking, but w/e (and if anyone catches any errors let me know).

    Edit: omg after previewing this, it is a wall...

    This may be overkill, but I started a website a while back to explain semiconductor devices, but never did anything with it. Maybe this write-up will be a guide for an article someday. Also, the last sentence pretty much sums up how an LED works, but everything else is a basic way of understanding how it is done. I noticed I didn’t go into much detail on some topics, so if something is confusing let me know. I really need a chapter (or a few) of a book to talk about this.

    The basic structure of the LED is the pn junction. This is a junction between a p-type and n-type semiconductor (making two materials physically touch). To understand what this means, I will first talk about silicon since that is what I learned about in my classes first. (Silicon does not make a good LED, but is good place to start for understanding the physics.)

    First, what is a semiconductor? What you need to know is that under certain conditions it can conduct or not conduct electric current. Silicon has 4 valence electrons, so in order to fill its shell it needs to bond to 4 other silicon atoms. In a crystal silicon has a diamond cubic structure.

    Now let’s say we have a perfect silicon crystal at absolute zero. There is no thermal energy (quantum theory says there has to be some motion, but we can ignore that here). Every silicon atom is bonded to 4 other atoms, so every atom has a complete outer shell. All of the electrons are in the “valence band.” All of the electrons are bound to their atoms, so no electrons are available for conducting current.

    Let’s increase the temperature: Thermal energy is just the motion of the atoms. As the temperature goes up, electrons gain energy. By chance, some electrons gain enough energy to break free from the atoms they were bonded to and they are now available to act as a negative charge carrier in the "conduction band." This means that they are used for conducting current since they are no longer stuck to one atom. At the same time, you can think of the empty state that was left behind in the valence band as a positive charge carrier, or a hole. If an electron to the right of it moves into its place (electron goes from right to left), the hole effectively moves to the right.

    An important term to know is "bandgap." The electron needs so much energy to move to the conduction band, and this means that there are some energy levels that the electron can't have. If it tried to go there, it would just return to the valence band. This is hard to describe without drawings, so here is an analogy I just made up: Say I represent an electron, and the planet represents the atom. On the ground I am at equilibrium, and am bound to the planet. If I jump, I gain potential energy and am at a new energy level, but I am not stable in the air and I return down to the ground. However, if I was able to jump really high and gain much more potential energy, I could escape the effects of gravity. I would be "free," and if I was the electron I would be in the conduction band. The point is that I cannot remain at the energy levels between me on the ground and me being free. They are forbidden, and there are forbidden energy levels for an electron (Someone let me know how this analogy is).

    As the temperature increases, more and more electrons gain enough energy to break away from their atoms, and the material becomes more and more conductive. As long as we are talking about pure silicon, the number of electrons that jump up to the conduction band equal the number of holes: each electron leaves behind a hole. This is known as an intrinsic semiconductor.

    Now to explain what p and n type is: Say we replace a silicon atom in the crystal with phosphorous (P). P has 5 electrons in its outer shell. After it bonds with 4 silicon atoms, it has an extra electron, and at room temperature that electron has enough energy to escape the P atom. It is now part of the conduction band (Since P donates an electron to the conduction band it is called a donor). As more P is added, more electrons are added to the conduction band. When this is done, it is called n-type: The impurity atom (P) donates an electron to the conduction band. Notice that these electrons do not leave behind holes, so now there are more electrons than holes. This is called an extrinsic semiconductor.

    A similar situation occurs if boron (B) replaces a silicon atom. However, B only has 3 valence electrons, and in order to have a full valence shell an electron from a silicon atom is used (Since B accepts and electron, it is called an acceptor). Now, there is a missing electron in the valence band. This is a hole, just like it was described earlier. Similar to n-type, the hole does not add an electron to the conduction band, so now there are more holes than electrons, and it is p-type.

    To recap, in n-type material electrons carry the most current since there are more electrons than holes, and the opposite is true for p-type. I also have to mention that electrons in the conduction band constantly keep falling back into the valence band and "recombine" with holes. This is because they lose energy via collisions with other particles. At the same time, electrons in the valence band can gain energy from collisions and move up to the conduction band, known as “generation.” At thermal equilibrium these happen at the same rate and it appears like there is a constant number of electrons and holes. For more on this, look up Fermi-Dirac statistics.

    Now we can look at what happens when an n-type and p-type material are brought into contact. There are many electrons in the n-type one, and few in the p-type. This means that there is a gradient in the electron concentrations across the junction. Just like when you spray cologne and it diffuses through the room (it moves from high concentration to low), electrons move from the n side to the p side. The same reasoning can be used for holes. (This generates an internal electric field that opposes the diffusion, and at equilibrium these two things balance, but this is not needed to understand the LED qualitatively.)

    This is where the LED comes in, and why silicon is a bad one. First, picture what happens when a bunch of electrons start moving one way and holes the other. They are going to pass each other, and in this region the number of electrons and holes is not at equilibrium (There is inversion). The electrons will then recombine with the holes a lot more than in regions that are at equilibrium (in the p and n material far from the junction). In silicon, the energy lost by the electron dropping from the conduction band to the valence band is given off as a phonon, or lattice vibration (heat). In certain types of materials that are called direct bandgap materials, the energy will be given off as a photon (light).

    This is what makes an LED shine: Injecting electrons and holes in opposite ways across a junction so that they can recombine and generate light.
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  5. #4  
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    Very nice explanation, I wouln't have had the patience to write it. Incidentally, although my formal education is Physics, my work involves Chemistry. I work for a company which produces Phosphine and almost a third of our business is high purity PH3 used in the LED industry as a dopant.
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  6. #5  
    Forum Freshman dcOSU's Avatar
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    Interesting, I am taking a lab course right now, and we are fabricating various microelectronic devices on a silicon wafer (see my avatar). We used phosphine gas for the diffusions in the transistors, diode, and resistors.

    And I rarely do write ups like that, I guess I will do anything to procrastinate my homework.
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  7. #6  
    Forum Freshman AlphaParticle's Avatar
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    dcOSU, that was an amazing and amazingly long post about LED, unfortunately, i' m 13 and ofcourse being adolescent couldn't be bothered to read the whole thing, but thankyou very very much for your time, in writing it, it's much apreciated!!
    AlphaParticle
    "An eye for an eye leaves everybody blind."
    - Martin Luther King Jr
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  8. #7  
    Forum Freshman dcOSU's Avatar
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    No problem, but I would appreciate it if you tried to understand it. I am curious if I did a decent job at explaining the concepts as simple as possible, or if it just makes no sense to someone who hasn't studied in my field.
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  9. #8  
    Veracity Vigilante inow's Avatar
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    I think you may have spent a bit too much time on the wafer and channel side of things. Without some training and background on that, it seems a little too advanced. It's like explaining the mRNA pathway to a person who just heard the word genetics for the very first time, or sodium potassium channels to a person who didn't even know the body communicates internally through nerve cells.
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  10. #9  
    Administrator KALSTER's Avatar
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    sodium potassium channels to a person who didn't even know the body communicates internally through nerve cells.
    It does?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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  11. #10  
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    Nope, small fairies run around with notes :P
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  12. #11  
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    bandgap energy = freq of light produced?
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